Buried pipelines are often coated and/or subjected to cathodic protection techniques in an effort to mitigate corrosion. Cathodic protection may be achieved by electrically connecting the pipeline to another more easily corroded metal to act as the anode of a simulated electrochemical cell. Another type of cathodic protection technique uses impressed currents; a current is applied to the pipeline to force the pipeline surface to act as a cathode.
However, even the best of today's corrosion avoidance techniques do not prevent corrosion at some point. In the pipeline industry, defects in the pipeline coating are known as “holidays”, and result in the metal surface of the pipeline being exposed to the soil. Furthermore, coatings can degrade and exfoliate, creating areas where the coating material is present but partially or wholly disbonded from the pipe, limiting protection from corrosion.
Knowledge of in-situ corrosion rates at holidays and disbonded coating sites is important to integrity management of buried pipelines. However, buried pipelines are not easily accessible for corrosion detection. Thus, special techniques have been developed for detecting coating defects and other corrosion without need to expose the pipe surface.